Carburizing is a thermochemical process for increasing the surface hardness of low-carbon, low-alloy or unalloyed steels, by increasing the carbon content in the exposed surface of steel. Carburizing is typically accomplished by exposing a steel article to a carbonaceous atmosphere containing carbon in an amount greater than the carbon content of the steel, and heating the steel article to a temperature above its austenite transformation temperature. After a desired amount of carbon has been diffused into the steel surface, and a desired carburized case depth has been attained, hardness is induced by quenching.
Gas carburizing is a widely used method for carburizing steel. Being a diffusion process, carburizing is affected by the amount of alloying elements in the steel composition and the carburizing process parameters such as the carbon potential of the carburizing gas, the carburizing temperature and the carburizing time.
During carburizing, the primary purpose is to provide a hardened layer or a case, having a higher carbon content than the core, and a definite depth. Besides the formation of a carbon-rich case on the steel surface, surface oxides are inadvertently formed by the reaction of small amounts of oxygen in the carburizing gas with the alloying elements in steel, such as silicon, manganese and chromium. These surface oxides form in an intergranular manner, i.e., they tend to form along the grain boundaries, extending inwardly from the surface. It is known that the formation of surface oxides have a detrimental effect on the mechanical properties of the carburized steel article, such as bending fatigue strength.
In the past, researchers have investigated the effects of lowered levels of silicon on the reduction of intergranular oxides. It is generally known that restricting the amount of silicon in the steel composition is beneficial for reducing the formation of intergranular oxides. An article authored by Ruth Chatterjee-Fischer, titled "Internal Oxidation During Carburizing and Heat Treating", Metallurgical Transactions A, published by American Society for Metals and the Metallurgical Society of AIME, Vol. 9A, Nov. 1987, pp. 1553-60, concluded that the silicon content in steel significantly affects surface oxidation. However, this article fails to recognize any relationship between reduced silicon content and the enhancement of carburized case depth.
A low silicon carburizing steel was developed by Kobe Steel, Ltd., and described in a Japanese Patent Publication No. 61-253346. To assure favorable surface hardenability and prevent the formation of granular oxides, Kobe limited the amounts of chromium, manganese, silicon and molybdenum to minimize the formation of surface carbides and oxides. However, although the Kobe reference limits the amount of silicon to less than 0.10% by weight to prevent oxide formation, it does not disclose the effect of reduced silicon content on the enhancement of carburized case depth.
Another low silicon carburizing steel was developed by Sanyo Special Steel Co., Ltd., and described in a Japanese Patent Publication No. 57-23741. To accelerate carbon diffusion and assure a faster carburizing time, Sanyo limited the amount of silicon in the steel composition to within a range of 0.06% to 0.12% by weight, in combination with relatively higher amounts of chromium and carbon. However, Sanyo concluded that only decreasing the silicon content by itself does not enhance the carburized case depth in a given time. Further, Sanyo also concluded that the effect of reducing the silicon content on accelerating the carbon diffusion becomes saturated, or minimum, at about a 0.06% silicon content. Contrary to this conclusion and quite unexpectedly, it has been discovered in the present invention, that an enhancement of the carburized case depth is achieved at even lower silicon levels, of below 0.05%, without a decrease in the mechanical properties.
U.S. Pat. No. 4,921,025 issued May 1, 1990 to Tipton et al., and assigned to the same company as this instant invention, discloses a process which uses a low silicon steel having no more than 0.10% silicon, for forming a carburized steel article which is free of surface intergranular oxides and has a high percentage of surface carbides. Although the Tipton patent is very useful for forming steel articles having enhanced resistance to bending loads, surface wear and contact fatigue, Tipton does not suggest exploiting the advantages of further reducing the amount of silicon to result in a significantly reduced carburization time.
It is desirable to have a carburized steel article that demonstrates a combination of an improved response to carburization, and a surface microstructure that is substantially free of intergranular oxides. In addition, it also desirable that after carburizing, the steel article does not require any removal of material from the carburized surface. It is further desirable that the steel article be able to resist a combination of bending loads, surface wear and contact fatigue. Further, it is extremely desirable to provide a carburizing process which will yield a substantial increase in the carburized case depth for a fixed carburization time. The present invention is directed to overcome one or more of the problems as set forth above.